As the filmmakers not having faith in this project being able to stand up on its own. That it needs all these superfluous LoTR tie-ins. That instead of being its own story, it needs to be a LoTR sequel (assuming Frodo is reading the book after the events of LoTR).

Does The Hobbit need to be told through Frodo's reading of Bilbo's book? Not in the least bit.

Now The Hobbit has to be viewed in the context of The Lord of the Rings, instead of it being the other way around.

Remember, from a storyline perspective, The Hobbit doesn't need The Lord of the Rings. The Lord of the Rings needs The Hobbit.